Process of electrolytic parting



Patented June 29, 1943 2,322,786 PROCESS OF ELMTROLYTIC PABTING Jesse 0. Betterton, Metuchen, Albert J. Phillips, Plainfield, and Henry B. Linford, Woodbrldge,

N. 1., asslgnors to American Smelting and Re- Company, New York, N. Y., a corporation of New Jersey No Drawing. Application February 11, 1939, Serial No. 255,868

6 Claims. 204-109) The process of the present invention provides certain improvements in the practice heretofore regarded as standard for recovering precious metals contained in anode slimes from electrolytic copper refineries; and more particularly, the present process provides certain improvements in the electrolytic parting of silver from gold, it dealing, more particularly, with modifications of the generally accepted practice having for their broad purpose the effecting of improved economies in operation thereof, particularly with reference to the preparation of electrolytic solutions, and to the handling of the gold slimes resulting from the electrolytic decomposition of the dor anodes produced from the treatment of the aforesaid anode slimes.

The dor anodes are produced from the residues left from the leaching of the anode slimes resulting from electrolytic copper refining, such residues being importantly high in precious metals which are subject to recovery by a reverberatory refining with soda ash and niter, and casting the resulting dor metals into anodes. These -anodes are electrolyzed in suitable cells, for' example cells of the well-known Moebius type, utilizing a silver nitrate electrolyte produced by solution of a portion of the dor anodes in nitric acid. During the course of this electrolysis, silver is deposited on the cathodes as fine silver, and the gold from the anodes forms slimes which are collected in suitable bags enclosing the anodes, and which, after a suitable treatment, are cast into anodes and electrolytically refined in Wohlwill cells in a gold chloride-hydrochloric acid electrolyte for'the deposition of substantially pure gold on the cathodes of the cells.

During electrolysis of the dor anodes, there must be replenishment of the electrolyte in the cells from time'to time, as required by the depletion of electrolyte through evaporation and deposition of the silver content thereof; this replenishment being accomplished by the addition of further quantities of silver nitrate solution as make-up electrolyte for the cells.

Also, the gold slimes which are collected during the decomposition of .the anodes are subject to treatment for separation therefrom of entrained impurities, mostly silver, before the slimes are melted out and cast as Wohlwill anodes which are refined, subsequently, to substantially pure gold in Wohlwill cells.

The present invention provides improvements in the carrying out of the electrolysis under closely controlled conditions which are found to enhance the purity of the recovered silver, with consequent diminution of accompanying losses of values, particularly precious metals other than the gold and silver present in the dor anodes.

It may be pointed out in this connection that copper tank house slimes, from which the dor anodes are produced, may contain important The present process includes, accordingly, an

improved operating procedure whereby such palladium losses are reduced to a very substantial extent, and frequently substantially entirely eliminated.

In accordance with the present process, dor anodes which may have been produced from refined leached anode slimes from electrolytic copper refining operations, are electrolyzed in Moebius cells using as electrolyte, a solution of neutral silver nitrate, which solution may be produced by dissolving suitable quantities of the dor metal in nitric acid.

As a result of theelectrolysis, silver from the anodes is deposited as fine silver crystals, which in practice are scraped from the cathodes as deposited, so that these silver crystals settle to the bottom of the cells, where they collect.

The gold and other precious metals present in the anodes are collected as slimes in bags which enclose the anodes, and as the anodes become consumed, the resulting scrap is melted and recast, and electrolyzed again as anodes.

The fine silver crystals which are collected at the bottom of the cells are washed in hot water, preferably in filter bottom trucks, the bulk of the resulting wash water which contains silver nitrate electrolyte from the silver crystals being returned to the storage tank for addition to the electrolytic cells as required. Excess quantitles of wash water are sent to a cementation tank, where the silver present in the wash water is precipitated by the addition of a suitable cementation metal such as zinc.

The liquid from the cementation operation is separated from the metallic precipitate and the solution is discarded, the precipitated metal being cupelled to silver.

The fine silver crystals recovered from the electrolytic cells are melted and cast into bars, there being reserved, however, a suitable, but

relatively small quantity 'of such crystals for use in preparing further quantities of silver nitrate electrolyte which is to be substantially neutral withrespect to free nitric acid.

This neutral silver nitrate solution is returned to the storage tank and mixed with the wash water from the washing operation on the silver crystals, the resultingsolution being returned to the electrolytic cells as make-up solution as is required in accordance with depletion of the w electrolyte in the electrolysis.

In operating the electrolysis in accordance with the usual practice, it has been observed that if the dor anodes contain palladium, the fine silver may contain important amounts of palladium which is deposited along with the silver.

One aspect of the present invention is to provide an improved procedure of electrolysts of the dor anodes which will eliminate excessive losses of palladium in the fine silver.

It has been found that the higher the pH value of the electrolyte (that is, the lower the free nitric acid concentration) the lower becomes the palladium content of the fine silver deposited, values of less than 0.001 part palladium per thousand of silver being obtainable, if the pH of the electrolyte is maintained at a value greater than 3.0, but less than what would cause precipitation of copper andsilver salts. In prac- Palladium in line silver in parts per 7 thousand wws s we"? I ages SW82 It will be seen from the above that there is a definite decrease in the palladium content to a pH value of 3.0, and while further slight decreases are noted, with increasing values of pH. these decreases are very slight and may arise the pH value increases. For practicable commercial operation, a-current. density range of from substantially tit ampsJ/sd. ft. is found to be the best operating range for maintaining a pH of requisite range of 2.8 to 3.8 in

the electrclyte,-it being found in practice that the current. density should not be increased above amps/sq. it.

It will be understood, of course, that the current density in itself does not affect directly the palladium content of line silver. the current density efiect on such content being exerted only through its efiect on the pH of the electrolyte.

What is claimed is:

1. Process for electrolytic parting of I silver from dor anodes which comprises preparing electrolyte from silver-bearing products of electrolysis of the anodes by dissolving silver from the said products in controlled amounts of nitric acid to avoid the presence of substantial amounts of free nitric acid in the electrolyte, and electrodepositing the silver from the said electrolyte while maintaining the pH value of the electrolyte at approximately 3.0, thereby avoiding important inclusions of palladium in the deposited silver.

2.. Process for electrolytic parting of silver from dor anodes which comprises preparing electrolyte from silver-bearing products of electrolysis of the said anodes by dissolving the said products in controlled amounts of nitric acid to avoid the presence of substantial amounts of free nitric acid in the electrolyte, and electrodepositing fine silver from the electrolyte while maintaining the pH value of the electrolyte between 2.8 and 3.8 to avoid the formation of larg amounts of free acid in the electrolyte as well as precipitation of through the limits of analytical accuracies as ditionally, it has been noted that increases of pH value substantially above 3.0 results in an accompanying precipitation of basic salts of copper, which occurs with a pH of about 4.2, and of basic salts of silver, which occurs with a pH of 5, in the electrolyte,. so that a pH of approximately 3.0 may be taken safely as the critical limit of practical operativeness.

It will be understood, of course, that the process of the present invention is applicable to pal.- ladium-containing dor anodes from any source, such as for example, from lead refining operations, as well as from copper tank-house refining processes.

During the course of the investigations,'inci-' dent to the present invention, it has been found that the requisite pH value may be maintained by controlling the anode current density so as not to exceed a definite maximum, and that with any given current density the pH of the electrolyte comes to an equilibrium. It has been found that, starting with an electrolyte of-p H=3, an increase of current density of 40 amps/sq. ft. or more, the pH value goes down; whereas with basic salts therein and thereby excluding important amounts of palladium from the deposited silver.

3. Process of electrolytic parting of silver from dor anodes which comprises maintaining the silver nitrate electrolyte continuously at a pH value below 4.2 to avoid precipitation of basic compounds in the electrolyte during electrolysis and above 2 to avoid building up of substantial amounts of free acid in the electrolyte and inclusion of important amounts of palladium in the deposited silver.

4. Process of electrolytic parting of silver from dor anodes which comprises electrolyzing the anodes in a silver nitrate electrolyte while continuously maintaining the pH of the electrolyte a current density of less than 40 amps/sq. it, 7

between 2.8 and 3.8 to substantially eliminate inclusions of palladium in the deposited silver.

5; Process of electrolytic parting of silver from dor anodes which comprises electrolyzing the anodes in a silver nitrate electrolyte while regulating anode current densities between such limits as will maintain continuously th pH of the electrolyte between 2.8 and 3.8 to subtantially eliminate inclusions of palladium in the deposited silver.

6. Process of electrolytic parting of silver from dor anodes which comprises electrolyzing the anodes in a substantially neutral silver nitrate 

